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cpu.cpp
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cpu.cpp
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#include <iostream>
#include <iomanip>
#include <cstdlib>
#include <ctime>
#include "cpu.h"
#include "graphics.h"
#include "memory.h"
namespace Chip8 {
const uint16_t kOpCodeMask = 0xF000;
const uint16_t kAddressMask = 0x0FFF;
const uint16_t kRegisterXMask = 0x0F00;
const uint16_t kRegisterYMask = 0x00F0;
const uint16_t kImmediateMask = 0x00FF;
const uint16_t kLastNibble = 0x000F;
const SDL_Keycode kKeyCodeMap[16] = {
SDLK_0, SDLK_1, SDLK_2, SDLK_3,
SDLK_4, SDLK_5, SDLK_6, SDLK_7,
SDLK_8, SDLK_9, SDLK_a, SDLK_b,
SDLK_c, SDLK_d, SDLK_e, SDLK_f,
};
CPU::CPU(Graphics* g, Memory* m) : _g(g), _m(m) {
std::srand(std::time(0));
reset();
}
void CPU::reset() {
_program_counter = 0x200;
_delay_timer = 0;
_sound_timer = 0;
_stack_pointer = 0;
_index_register = 0;
std::fill_n(_registers, sizeof(_registers), 0);
}
void CPU::run_cycle() {
// Decode instruction
OpCode op = _m->getByte(_program_counter) << 8 | _m->getByte(_program_counter + 1);
std::cout << "Operation at 0x"
<< std::hex << _program_counter << " -> " << std::setw(4) << int(op)
<< std::endl;
Address a = op & kAddressMask;
uint8_t rx = (op & kRegisterXMask) >> 8;
uint8_t ry = (op & kRegisterYMask) >> 4;
uint8_t last_byte = op & 0x0FF;
// Run instruction
switch (op & kOpCodeMask) {
case 0x0000:
if (op == 0x00E0)
// Clear the screen
_g->clear();
else if (op == 0x00EE)
// Return from subroutine
_program_counter = _stack_pointer;
else
_m->dump();
break;
case 0x1000:
_program_counter = a;
break;
case 0x2000:
_stack_pointer = _program_counter + 2;
_program_counter = op & 0x0FFF;
break;
case 0x3000:
if (_registers[rx] == (op & kImmediateMask))
_program_counter += 4;
else
_program_counter += 2;
break;
case 0x4000:
if (_registers[rx] != (op & kImmediateMask))
_program_counter += 4;
else
_program_counter += 2;
break;
case 0x5000:
if (_registers[rx] == _registers[ry])
_program_counter += 4;
else
_program_counter += 2;
break;
case 0x6000:
_registers[rx] = (op & kImmediateMask);
_program_counter += 2;
break;
case 0x7000:
_registers[rx] += (op & kImmediateMask);
_program_counter += 2;
break;
case 0x8000:
switch (op & kLastNibble) {
case 0x0:
// Sets VX to the value of VY
_registers[rx] = _registers[ry];
break;
case 0x1:
// Sets VX to VX OR VY
_registers[rx] |= _registers[ry];
break;
case 0x2:
// Sets VX to VX AND VY
_registers[rx] &= _registers[ry];
break;
case 0x3:
// Sets VX to VX XOR VY
_registers[rx] ^= _registers[ry];
break;
case 0x4: {
// Adds VY to VX, VF is set to carry
uint16_t sum = _registers[rx] + _registers[ry];
_registers[0xF] = (sum > 0xFF) ? 1 : 0;
_registers[rx] = sum % 0xFF;
break;
}
case 0x5:
// VY is subtracted from VX, VF is set to zero when there is a borrow
_registers[0xF] = (_registers[ry] > _registers[rx] ? 0 : 1);
_registers[rx] -= _registers[ry];
break;
case 0x6:
// Shifts VY right by one and stores the result in VX, VF set to least
// significant bit before shift
_registers[0xF] = _registers[ry] & 0x0001;
_registers[rx] = _registers[ry] >> 1;
break;
case 0x7:
// Sets VX to VY minus VX, VF is set to zero when there is a borrow
_registers[0xF] = (_registers[rx] > _registers[ry] ? 0 : 1);
_registers[rx] = _registers[ry] - _registers[rx];
break;
case 0xE:
// Shifts VY left by one and stores result in VX, VF set to most
// significant bit before shift
_registers[0xF] = (_registers[ry] & 0x8000) >> 8;
_registers[rx] = _registers[ry] << 1;
break;
}
_program_counter += 2;
break;
case 0xA000:
_index_register = (op & kAddressMask);
_program_counter += 2;
break;
case 0xC000:
_registers[rx] = std::rand() & (op & kImmediateMask);
_program_counter += 2;
break;
case 0xD000: {
int x = _registers[rx];
int y = _registers[ry];
int n = (op & kLastNibble);
uint16_t sprite_pointer = _index_register;
bool xored = false;
// Height is determined by the last nibble
for (int i=0; i < n; i++) {
// Chip8 sprites are ALWAYS 8 pixels wide
uint8_t sprite_mask = 0x80;
uint8_t sprite_row = _m->getByte(sprite_pointer + i);
//std::cout << "[cpu] Current sprite pointer: " << (int)sprite_pointer + i << std::endl;
//std::cout << "[cpu] Current sprite row: " << (int)sprite_row << std::endl;
for (int j=0; j<8; j++) {
if (sprite_row & sprite_mask) {
bool on = _g->get(x + j, y + i);
if (on) xored = true;
_g->set(x + j, y + i, !on);
}
sprite_mask >>= 1;
}
}
_registers[0x0F] = xored;
_program_counter += 2;
break;
}
case 0xE000:
switch (last_byte) {
case 0xA1: {
// Skip the following instruction if the key corresponding to the hex value
// currently stored in register VX is not pressed.
uint8_t key = _registers[rx];
SDL_Event event;
while (SDL_PollEvent(&event)) {
if (event.type == SDL_KEYDOWN) {
if(event.key.keysym.sym == kKeyCodeMap[key]) {
std::cout << "skipping" << std::endl;
_program_counter += 2;
}
}
}
}
}
_program_counter += 2;
break;
case 0xF000:
int mod = 100;
switch (last_byte) {
case 0x07:
// Store the current value of the delay timer in register VX
_registers[rx] = _delay_timer;
break;
case 0x15:
// Set the delay timer to the value of register VX
_delay_timer = _registers[rx];
break;
case 0x29:
// Set index register to location of font for hex digit in VX
_index_register = _m->getFontLocation() + _registers[rx];
break;
case 0x33:
// Store the binary-coded decimal equivalent of the value stored in
// register VX into subsequent memory addresses starting at the address
// currently in the index register.
for (int i=0; i<3; i++) {
_m->putByte(_index_register + i, _registers[rx] % mod);
mod /= 10;
}
break;
case 0x65:
// Fill registers V0 to VX inclusive with the values stored in memory
// starting at the address currently in the index register.
for (int i=0; i<=rx; i++)
_registers[i] = _m->getByte(_index_register + i);
break;
// TODO: set index register to index register + x + 1 after this?
}
_program_counter += 2;
}
// Lets check out what is going on inside the registers of the CPU
dump();
// Decrement the timers -- because the timers run at the same clock rate as the CPU
// itself (60hz) this is totally fine.
// TODO: skip decrement on cycle when timers are set
if (_delay_timer > 0) {
_delay_timer--;
}
if (_sound_timer > 0) {
_sound_timer--;
}
}
void CPU::dump() {
std::cout << "-------- CPU Registers --------" << std::endl;
for (int i=0; i<4; i++) {
std::cout << std::hex << std::setfill('0') <<
std::setw(1) << i + 0 << ": 0x" << std::setw(2) << int(_registers[i+0]) << " " <<
std::setw(1) << i + 4 << ": 0x" << std::setw(2) << int(_registers[i+4]) << " " <<
std::setw(1) << i + 8 << ": 0x" << std::setw(2) << int(_registers[i+8]) << " " <<
std::setw(1) << i + 12 << ": 0x" << std::setw(2) << int(_registers[i+12]) << " " <<
std::endl;
}
std::cout << std::endl;
std::cout << "pc: 0x" << std::setw(3) << int(_program_counter) << std::endl
<< "ix: 0x" << std::setw(3) << int(_index_register) << std::endl
<< "sp: 0x" << std::setw(3) << int(_stack_pointer) << std::endl
<< "dt: 0x" << std::setw(2) << int(_delay_timer) << std::endl
<< "st: 0x" << std::setw(2) << int(_sound_timer) << std::endl;
std::cout << "-------------------------------" << std::endl;
std::cout << std::endl;
}
} // namespace Chip8